I'm looking for a word or term that describes the way poeople use familial words to describe their friends, such as: "brother" or "sister" (while they're not really brothers and sisters from the same father or mother). Can I use the word "nickname" in this context?
I need to use this term in such sentence:
Person A: This is my brother.
Person B: Nice to meet you.
Person C (not in front of person A): Is he really his brother or it's (=the word brother) just a (nickname? "affection word" / "fondness word"?)
I'm looking for (a) word/s to describe the usage of such words (brother & sister) in a context of friendship. "I didn't really mean that he's my brother, I used the word brother as a... (nickname?)"